Membrane techniques are known separation processes used both at the molecular and ionic levels. Such processes consume little energy and enable the concentration and fractionation of milk. Energy is saved for instance because the use of membrane techniques does not require a phase change in dewatering as do condensation and evaporation.
Generally speaking, membrane filtration processes of four basic types are in use, each of which serves a different purpose of use. According to the separation ability, these four basic types are, from the one having the smallest pore size to the one having the largest pore size, reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF) and microfiltration (MF). Of these, reverse osmosis is generally used for concentration, ultrafiltration and microfiltration for fractionation, and nanofiltration for both concentration and fractionation.
Microfiltration, ultrafiltration and nanofiltration are membrane separation processes, wherein liquid is filtered through a semi-permeable membrane. A semi-permeable membrane is a membrane that lets through only part of the components in a solution. The system may also comprise a preliminary filter for filtering off the largest or precipitated components.
Osmosis is the spontaneous movement of a liquid through a semi-permeable membrane from a dilute solution through the membrane to a more concentrated solution. In a reverse osmosis device, the flow is reversed by an increase in the pressure of the concentrated solution to exceed the osmotic pressure. Reverse osmosis enables the separation of the dissolved salts. In practice, the liquid (permeate) obtained from reverse osmosis is pure enough to be discharged in a sewer. The most common use of reverse osmosis is the production of drinking water from seawater.
The use of membrane techniques enables the separation of milk components by bringing milk to flow at a raised pressure through a membrane. The components that are smaller than the pore size of the membrane will then pass through the membrane (permeate) and the larger components are retained behind the membrane (retentate). In other words, two flows that leave the separation system are always generated.
During the last decades, the dairy industry has successfully used membrane techniques for instance in the treatment of whey and wastewater. However, an observation has been made in the dairy industry that membrane techniques are extremely well suitable for the treatment of cow's milk, which is known to contain abundant amounts of valuable nutrients and functional compounds. Recent studies have in fact concentrated on the membrane filtration of milk and the use of such filtered milk in the production of dairy products, such as cheese, ice cream and yoghurt.
Special attention has been paid in the studies to the increasing demand for lactose-free milk products during the last few years. It is generally known that some individuals are intolerant to lactose, i.e. cannot tolerate milk products containing a normal amount of lactose. In addition, sometimes it is necessary to ingest low-lactose milk products for some other reason. For example, when an individual has taken antibiotics, the intestinal ability to break down lactose into monosaccharides may be impaired.
Several processes have been presented for eliminating lactose from milk. Generally speaking, the problem in all known processes is a change in the organoleptic characteristics. A well known process in the field is the conventional enzymatic process of eliminating lactose, the process comprising the step of adding lactase into milk, thus resulting in the conversion of more than 80% of the lactose into monosaccharides, i.e. glucose and galactose. Here, the problem is the unacceptably sweet taste of the milk, caused by the monosaccharides.
WO publication 00/45643 discloses a process that aims at retaining the organoleptic characteristics of milk. According to the publication, this is achieved by reducing the amount of lactose so as to reach a lactose to protein ratio of about 1:1, and then treating the milk with lactase in order to convert the residual lactose into monosaccharides. According to the publication, the amount of lactose can be reduced either by ultrafiltration or diafiltration. The essential characteristic of the process presented is the reduction of the lactose to protein ratio. This is also achieved by increasing the amount of protein either by concentrating the original milk or by adding protein into the milk in any process step. The problem in such a process is that in association with ultrafiltration or diafiltration, not only lactose, but also part of the salts that have a clear significance for the taste of milk are also eliminated from the milk. Another hindrance to the use of ultrafiltration is the difficulty in utilizing the by-product, permeate, which contains water, lactose, salts and low-molecular nitrogen compounds.
Finnish publication 104 783 B1 discloses a process for preparing a whey salt powder from whey or a permeate from milk ultrafiltration. The process comprises nanofiltration, concentration and drying of the whey or the permeate. The whey salt powder obtained by the process is usable as a substitute for conventional table salt (NaCl).
EP publication 226 035 B1 discloses a process for specific chromatographic separation of lactose from milk. In the process, milk is fractionated in such a manner that a lactose fraction is separated and the salts remain in the protein fraction or the protein/fat fraction. The advantage of the process is that instead of a permeate, a pure lactose solution is obtained and that all substances significant to the taste, including salts, remain in the milk. However, chromatographic separation is a time-consuming and complex process. Another problem in chromatographic separation is the high purchase price of the equipment, since conventional dairies do not usually have such equipment.